Frost protection mechanism based on rubber airbag interlayered composite lining system for cold-region tunnels

To solve the problem of freezing damage in cold-region tunnels, this study proposed a rubber airbag interlayered composite lining system, and tested its performance of buffering, pressure adjustment, waterproofing, and heat preservation by simulating a low-temperature environment in an artificial freezing chamber. The experiment results show that the frost-heaving force exerted on the lining can always be lower than 1.69 kPa by constantly adjusting the airbag pressure, and the maximum frost-heaving force can reach 28.25 kPa without the airbag. In addition, the airbag also has good waterproof performance. Finally, the airbag can significantly improve the temperature field of the surrounding rock and reduce the freezing depth (6.75 cm < 17.25 cm). The insulation effect of the airbag is positively correlated with its thickness and negatively correlated with the thermal conductivity of the filling gas. The insulation effect of CO2 is better than that of air. The rubber airbag interlayered composite lining system provides a new scheme for freezing damage control in cold-region tunnels.


Experimental preparation and methods
The rubber airbag interlayered composite lining system proposed in this paper for cold-region tunnels is a novel lining structure that incorporates a rubber airbag structural layer between the initial support and secondary lining, as shown in Fig. 1.The rubber airbag structural layer, which is composed of airtight hollow rubber bag units interconnected by their edge irregularities, uses water-expanding rubber in the irregular regions, as shown in Fig. 2. The airbags are connected to an air supply system, including inflation and deflation devices and monitoring instruments.They are filled with compressible gas with low thermal conductivity, and the extrusion forces from the internal pressure and water expansion pressure seal the joint gaps in a watertight manner.This structural layer provides pressure cushioning, monitoring, adjustment, waterproofing, and insulation functions, effectively addressing cold-region tunnel frost-related issues from multiple dimensions.It compensates for the shortcomings of traditional composite lining frost-heaving support and frost protection technologies and has considerable theoretical innovation and engineering application value.

Experimental models
Two experimental models were used: a circular tunnel with a rubber airbag interlayered structure (referred to as the "airbag model") and a circular tunnel without a rubber airbag interlayered structure (referred to as the "airbag-less model").Figures 3 and 4 present these models.The geometric similarity ratio of the models was 1:30, and the dimensions of the models were 300 mm (tunnel diameter) × 2400 mm (tunnel length).

Experimental equipment
The temperature of the freezing chamber, the boundary temperature of the surrounding rock, and the water temperature were automatically controlled by a programmable logic controller control cabinet with an accuracy   Temperature measurement was performed using WZP-50AA waterproof thermal resistance temperature sensors with a resolution of 0.01 °C.Temperature data were acquired using an R70B data logger.Precise pressure gauges were used for measuring the pressure of the rubber airbag interlayered structure with a resolution of 0.30 kPa, and pressure data were visually collected.The frost-heaving force was measured using an HNY-1 pressure sensor with a resolution of 0.30 kPa.A TST3826F dynamic-static strain testing and analysis system was employed for frost-heaving force data acquisition.

Experimental materials
The secondary lining (referred to as the "inner steel cylinder" in the model) was made of low-carbon steel sheets with a thickness of 1.5 mm.It had a diameter of 300 mm and a length of 2400 mm, as shown in Fig. 6a.The rubber airbag was made of synthetic rubber, forming a sealed hollow cylindrical structure.Water-expanding rubber components were mixed into both ends of the airbag.The airbags were available in three specifications: 30, 40, and 50 mm.The thickness of the bag wall was 2 mm, and the thickness of the bag cavity was 26, 36, and 46 mm, respectively.The rubber airbag had an inner diameter of 306 mm and a length of 2400 mm, as shown in Fig. 6b.The inflation gas inside the rubber airbag interlayered structure was either air or CO 2 gas, as shown in Fig. 6c and d, respectively.At 0 °C, the thermal conductivity (λ) of air was 0.024 W/(m•K), and for CO 2 gas, it

Buffering and pressure adjustment tests
The frost-heaving support function of the rubber airbag interlayered composite lining system primarily involves its reduction of the frost-heaving force of the surrounding rock.This is achieved through the pressure buffering and adjustment functions of the rubber airbag interlayered structure.The influence of the buffering function on the frost-heaving force is examined by adjusting the initial pressure of the airbag to alter the restraint state of the surrounding rock.During the freezing of the surrounding rock, the pressure was released promptly by deflating the airbag to study its effect on the frost-heaving force.The objective was to investigate the frost-heaving support mechanism and the effectiveness of the rubber airbag interlayered composite lining system.In contrast, the airbag-less model lacked the pressure adjustment function and did not reduce the frost-heaving force of the surrounding rock.This led to the accumulation of a significant amount of frost-heaving force.A comparison test was performed using the airbag-less model under the same conditions to highlight the functional advantages of the rubber airbag interlayered composite lining system.The test conditions designed according to the above ideas are presented in Table 1.
(1) Test conditions (2) Test sequence (1) A buffering test was conducted to examine the influence of the rubber airbag interlayered structure's buffering function on the frost-heaving force.(2) A pressure adjustment test of the rubber airbag interlayered structure was conducted to examine its influence on the frost-heaving force.Additionally, the frost-heaving force was measured for the airbag-less model under the same conditions.(3) Experimental procedures (1) Setup and calibration of measurement devices: For the airbag model, the frost-heaving force measurement instrument was installed on the inflation line of the rubber airbag to measure the equilibrium value of the surrounding rock's frost-heaving force, as shown in Fig. 7.For the airbag-less model, the frost-heaving force measurement instruments were arranged in four directions (A, B, C, D) on the cross-section of the model's surrounding rock.The average value of the frost-heaving force was calculated, as shown in Fig. 8.The specific procedure involved inserting the pressure sensor through small holes in the secondary lining (inner steel cylinder) to contact the base point of the surrounding rock.The sensor was securely supported by a bracket, and waterproofing was applied at this location.Before the start of the experiment, calibration and initial reading recording of the instruments and acquisition devices were performed to ensure the accuracy of the test data.(2) The water temperature in the water tank was adjusted according to the boundary temperature values of the surrounding rock, and then water was injected into the model.(3) Once the surrounding rock reached the design moisture condition, with the heating element in the model not powered, the temperature in the freezing chamber was reduced to the boundary temperature of the surrounding rock.The model was maintained at this temperature for an adequate period.(4) When the temperature of the model's surrounding rock (including the water) reached the boundary temperature value and the temperature distribution was uniform, the temperature of the heating element was set to the boundary temperature of the surrounding rock.Then, the power supply was connected to the heating element to maintain a constant boundary temperature of the surrounding rock throughout the test, using the heating element and insulation layer.(5) The temperature of the freezing chamber was set according to the design conditions, and the freezing test was initiated.In the airbag model test, the air in the freezing chamber exchanged heat with the secondary lining-the rubber airbag interlayered structural layer and the surrounding rock.In the airbag-less model test, the air in the freezing chamber exchanged heat with the secondary lining and the surrounding rock.( 6) For the buffering function test on the frost-heaving force, the frost-heaving force was measured after the freezing of the surrounding rock reached equilibrium.For the pressure adjustment function test of the frost-heaving force, frost-heaving force data were collected at intervals of 3 h starting from the beginning of the freezing of the surrounding rock.(7) After the experiment was completed, the operation of the test system was stopped, and inspection and closure procedures were conducted.

Waterproofing and frost protection tests
The frost protection effect of the rubber airbag interlayered composite lining system primarily manifests in the prevention of groundwater in the surrounding rock and thermal insulation of the rock.It relies on the waterproofing and thermal insulation functions of the rubber airbag interlayered structure.Tests were conducted to investigate the frost protection mechanism and effectiveness of the rubber airbag interlayered composite lining system, including tests of the waterproofing function, thermal insulation performance, and impact of thermal insulation on the temperature field of the surrounding rock.The airbag-less model without waterproofing or insulation is expected to result in severe frost damage.Thus, a comparison test was conducted using the airbagless model without waterproofing or insulation to highlight the advantages of the rubber airbag interlayered composite lining system for frost protection.The experimental conditions designed according to these considerations are presented in Table 2.
(1) Experimental conditions (2) Test sequence (1) A waterproofing test of the rubber airbag interlayered structural layer was conducted, and a seepage and freezing test of the airbag-less model was conducted under the same conditions.(2) A thermal insulation performance test of the rubber airbag interlayered structural layer was conducted.
(3) The impact of the thermal insulation function of the rubber airbag interlayered structural layer on the temperature field of the surrounding rock was tested, and simultaneously, the temperature field of the surrounding rock in the airbag-less model was measured under the same conditions.(3) Experimental procedures (1) Installation and calibration of measurement devices: The layout of the temperature measurement elements in the surrounding rock was identical between the airbag and airbag-less models.The measurement points were set in four directions (A, B, C, D) on the cross-section of the model's surrounding rock, with temperature measurement elements placed at intervals of 1.5 cm between measurement points.Additionally, temperature measurement elements were placed inside and on the outer side of the rubber airbag interlayered structural layer in the airbag model to measure the temperature difference between the inner and outer sides.Details are presented in Figs. 9 and 10.Before the start of the experiment, the temperature measurement elements and the acquisition system were calibrated to ensure the accuracy of the test data.(2) The water temperature in the water tank was adjusted according to the boundary temperature values of the surrounding rock, and then water was injected into the model.(3) Once the surrounding rock reached the design moisture condition, with the heating element in the model not powered, the temperature in the freezing chamber was reduced to the boundary temperature of the surrounding rock.The model was allowed to remain at this temperature for an adequate period.(4) When the temperature of the model's surrounding rock (including the water) reached the boundary temperature value and the temperature distribution was uniform, the temperature of the heating element was set to the boundary temperature of the surrounding rock.Then, the power supply was connected to the heating element, and the heating element and insulation layer were used to keep the boundary temperature of the surrounding rock unchanged throughout the test.(5) The temperature of the freezing chamber was set according to the design conditions, and the freezing test was initiated.In the airbag model test, the air in the freezing chamber exchanged heat with the secondary lining-the rubber airbag interlayered structural layer and the surrounding rock.In the airbag-less model test, the air in the freezing chamber exchanged heat with the secondary lining and the surrounding rock.(6) During the waterproofing test, the phenomenon of water seepage and freezing in the model was examined.In the thermal insulation performance test and the test of the impact of thermal insulation on the temperature field of the surrounding rock, after the freezing of the surrounding rock reached equilibrium, data on the temperature difference between the inner and outer sides of the rubber airbag interlayered structural layer and the temperature field of the surrounding rock were collected at measurement point A, for example.(7) The experiment was concluded, the operation of the test system was stopped, and inspection and closure procedures were conducted.

Results of the buffering and pressure adjustment tests
For the airbag-less model, the frost-heaving force caused by the low-temperature frozen volume expansion of the saturated water-containing surrounding rock acts directly on the secondary lining.For the airbag model, the frost-heaving force caused by the low-temperature frozen volume expansion of the saturated water-containing surrounding rock first acts on the airbag, and then transfers to the secondary lining.The frost-heaving force data from the buffering and pressure adjustment tests are presented in Tables 3 and 4.  The frost-heaving support effect of the rubber airbag interlayered composite lining system is mainly reflected in the positive effects of the cushioning function and pressure adjustment function of the rubber airbag interlayered structural layer on the frost-heaving force of the surrounding rock.
The results of the buffering test indicated that after the freezing equilibrium of the surrounding rock, the airbag model with an initial pressure of 18 kPa generated a frost-heaving force of 4.65 kPa, the airbag model with an initial pressure of 13 kPa generated a frost-heaving force of 2.63 kPa, and the airbag model with an initial pressure of 8 kPa generated a frost-heaving force of 1.37 kPa.A lower initial pressure of the rubber airbag interlayered structural layer corresponded to a weaker frost-heaving force generated by the surrounding rock.The pressure cushioning function of the rubber airbag interlayered structural layer significantly affected the frost-heaving force of the surrounding rock, as shown in Fig. 11.
The underlying mechanism is as follows: when the water-containing surrounding rock undergoes frostheaving deformation at low temperatures, it exerts pressure on the rubber airbag interlayered structural layer.Owing to the compressibility of the rubber material and the internal inflation gas, the rubber airbag interlayered structural layer relaxes the constraint on the frost heaving of the surrounding rock, changing the rigid constraint of the secondary lining on the frost heaving of the surrounding rock.Thus, the frost-heaving energy can cause the surrounding rock to undergo deformation and displacement toward the secondary lining.According to the principle of energy transformation through work done on an object, a lower initial pressure of the rubber airbag interlayered structural layer corresponds to a more relaxed frost-heaving constraint on the surrounding rock.The deformation and displacement of the surrounding rock caused by the frost-heaving energy will be larger, leading to more consumption of frost-heaving energy and effectively releasing the frost-heaving energy.This significantly weakens the frost-heaving force acting on the secondary lining and achieves the stability of the lining.
The results of the pressure adjustment test indicated that in the airbag model, during a freezing process lasting 45 h at an air temperature of -30 °C, the frost-heaving force was released by three instances of pressure relief, totaling 4.72 kPa.Eventually, the secondary lining bore no frost-heaving force (0 kPa).In contrast, the airbag-less model accumulated a frost-heaving force of 28.25 kPa under the same conditions, with the entire force acting on the secondary lining.The pressure adjustment function of the rubber airbag interlayered structural layer significantly affected the frost-heaving force of the surrounding rock, and this model exhibited outstanding functional advantages over the airbag-less model.
The underlying mechanism is explained as follows.Owing to the pressure adjustment function of the airbag model, it can actively intervene in the frost-heaving force during the freezing process through pressure adjustments of the rubber airbag interlayered structural layer.By conducting three instances of pressure relief, the frost-heaving force is consistently controlled within a certain range.The frost-heaving force curve exhibits a www.nature.com/scientificreports/wave-like fluctuation, eventually decreasing sharply to zero, indicating a complete release of the frost-heaving force.This provides excellent protection to the secondary lining.In contrast, the airbag-less model lacks pressure adjustment capability, resulting in the accumulation of a significant amount of frost-heaving energy.The trend of the frost-heaving force curve remains consistently high compared with the airbag model, which is highly detrimental to the secondary lining.Figure 12 shows the variation trends of the frost-heaving force in the airbag and airbag-less models during the pressure adjustment test.

Results of the waterproofing test
The test results for the waterproofing of the airbag and airbag-less models are shown in Fig. 13.
In the test, the airbag-less model exhibited severe water seepage and ice formation.This is because no waterproof measures were taken.When water infiltrated the secondary lining (inner steel cylinder) through fine pores in the surrounding rock, it froze at low temperatures, accumulating as ice at the bottom of the tunnel, as shown in Fig. 13a and b.
In contrast, the airbag model exhibited no water seepage or ice formation, demonstrating a significant waterproof effect.This is because when water infiltrated the rubber airbag interlayered structural layer within the surrounding rock, the impermeability of the rubber material prevented water from flowing toward the secondary lining (inner steel cylinder).When water infiltrated the gaps at the ends of the model, the inflation pressure inside the rubber airbag structural layer and the expansion and compression of the rubber material upon contact Frost-heaving force of surrounding rock (kPa) Airbag initial pressure (kPa) Frost-heaving force of surrounding rock with water sealed the gaps, preventing water from passing through.Therefore, no water seepage or ice formation occurred inside the tunnel, as shown in Fig. 13c.

Results of the insulation test
The temperature differences between the inner and outer sides of the rubber airbag structural layer with different thicknesses and different internal gases are presented in Tables 5 and 6, respectively.The test results indicated that under the same internal gas in the rubber airbag structural layer, the temperature difference between the inner and outer sides varied significantly with different thicknesses.The temperature difference was 9.26 °C for a thickness of 30 mm, 12.77 °C for a thickness of 40 mm, and 16.80 °C for a thickness of 50 mm.Similarly, under the same thickness of the rubber airbag structural layer, the temperature difference between the inner and outer sides differed significantly for different internal gases.The temperature difference was 9.26 °C for air and 15.22 °C for CO 2 .
The test results indicated that the rubber airbag structural layer had good thermal insulation performance.Additionally, a larger thickness and lower thermal conductivity of the internal gas corresponded to better insulation performance, as shown in Figs. 14 and 15.Therefore, in practical applications, the thickness and internal gas of the rubber airbag structural layer should be selected appropriately according to the temperature of the coldregion tunnel environment and the dimensions of the tunnel cross-section to ensure that the thermal insulation performance satisfies the requirements for insulation and frost protection in the tunnel.

Results of the surrounding rock temperature field test
The experimental data for the surrounding rock temperature field are presented in Tables 7 and 8.
The frost protection effect of the rubber airbag interlayered composite lining system is primarily reflected in its positive effects on the initial freezing time, temperature field distribution, and freezing depth of the surrounding rock through its thermal insulation function.The test results indicated the following.
Under the same conditions, the time at which the surrounding rock started freezing differed significantly between the airbag-less and airbag models.In the airbag-less model, freezing occurred after 16.2 h, whereas in  the airbag model, freezing occurred after 52.0 h.The surrounding rock in the airbag model froze later than that in the airbag-less model.After the freezing equilibrium of the surrounding rock was reached, the temperature at the measurement point 1 (base point) in the airbag-less model was -11.78 °C, at point 5 was -6.17 °C, at point 10 was -1.85 °C, that at point 15 was 1.54 °C, and that at point 17 was 2.66 °C.In the airbag model, the temperature at the measurement point 1 was -2.85 °C, at point 5 was -0.46 °C, at point 10 was 2.05 °C, at point 15 was 2.96 °C, and at   7. Initial freezing time of the surrounding rock in the airbag and airbag-less models.The freezing of the surrounding rock is marked by the appearance of 0 °C at the base point on the outer surface of the secondary lining/rubber airbag structural layer.The initial freezing time refers to the time interval between the appearance of 0 °C at the base point and the start of the freezing test.

Model Without With
Initial freezing time (h) 16.2 52.0 point 17 was 3.00 °C.Both models exhibited an increase in the temperature of the surrounding rock along the radial direction of the tunnel, but the temperatures at various measurement points were higher for the airbag model than for the airbag-less model, as shown in Fig. 16.
The position of the 0 °C line (freezing surface) after the freezing equilibrium of the surrounding rock was reached differed significantly between the two models.In the airbag model, it stopped between measurement points 5 and 6, whereas in the airbag-less model, it stopped between measurement points 12 and 13.This indicated that the maximum freezing depth of the surrounding rock in the airbag model was between 6.0 and 7.5 cm, which was significantly smaller than that (between 16.5 and 18.0 cm) in the airbag-less model.This is because the rubber airbag structural layer in the airbag model, which is filled with air, had a low thermal conductivity and high thermal resistance.It formed a thermal insulation layer between the model's surrounding rock and the secondary lining, which impeded heat transfer between the surrounding rock and the air inside the tunnel.This reduced the rate of temperature decline in the surrounding rock, delaying the onset of freezing.In addition, it inhibited the heat exchange between the surrounding rock and the air inside the tunnel, reducing the heat loss from the surrounding rock.This improvement in heat retention enhanced the distribution of the surrounding rock temperature field and reduced the depth of freezing in the surrounding rock.
Based on the present study results, the next step will be to study the frost protection performance of the rubber airbag interlayered composite lining system under different tunnel section types and surrounding rock porosity conditions.Table 8.Measurement results for the temperature field of the surrounding rock in the airbag and airbag-less models (direction A).

Measurement points
Depth of measurement (cm) The measured temperature of the airbag-less model (°C) The measured temperature of the airbag model (°C)

Figure 1 .
Figure 1.Cross section of the rubber airbag interlayered composite lining system.

Figure 2 .
Figure 2. Splicing method of the rubber airbag interlayered composite lining system.

Figure 4 .
Figure 4. Cross sections of the experimental models.(a) Airbag model; (b) Airbag-less model.

Figure 7 .
Figure 7. Monitoring method for the frost-heaving force in the airbag model's surrounding rock.

Figure 8 .
Figure 8.The layout of frost-heaving force measurement points in the airbag-less model's surrounding rock.

Figure 9 .
Figure 9.The layout of temperature measurement points in the surrounding rock of the airbag model.

Figure 10 .
Figure 10.The layout of temperature measurement points in the surrounding rock of the airbag-less model.

Figure 11 .Figure 12 .
Figure 11.Relationship between the frost-heaving force of the surrounding rock and the initial pressure of the airbag.

Figure 13 .
Figure 13.Comparison of waterproof performance between the airbag and airbag-less models.(a) Water seepage in the airbag-less model; (b) Ice formation in the airbag-less model; (c) No water seepage or ice formation in the airbag model.

Figure 14 .
Figure 14.The thermal insulation performance of the rubber airbag structural layer with different thicknesses.

2 Figure 15 .
Figure 15.The thermal insulation performance of the rubber airbag structural layer with different internal gases.

Figure 16 .
Figure16.Temperature field distribution law of the surrounding rock in the airbag and airbag-less models.

Table 1 .
The buffering and pressure adjustment tests for the surrounding rock.

Table 2 .
Frost protection test for the surrounding rock.For the waterproofing test, a small number of fine holes were drilled in the inner steel cylinders of both the airbag and airbag-less models to simulate the secondary lining joints.

Table 3 .
Results of the frost-heaving buffering test on the surrounding rock.

Table 4 .
Results of the frost-heaving force adjustment test on the surrounding rock.

Table 5 .
Temperature differences between the inside and outside of the structural layer of rubber airbags with different thicknesses.

Table 6 .
Temperature differences between the inside and outside of the rubber airbag structural layer with different internal gases.